The present invention generally relates to an apparatus with multiple actuated arms arranged in parallel for positioning and orienting an object in space with at least three degrees of freedom and retained inclination.
A need exists for simple and effective parallel robots, also known as parallel kinematics mechanisms. In general, robots are used in applications such as handling, assembling, positioning, picking and placing, packaging, palletizing, measuring, machining, and so on. Robots can be classified as serial or parallel. Serial robots, or serial kinematics mechanisms, are widely used and presently dominate the market.
A serial robot has a series of cantilever beams that are movably connected together in an end-to-end fashion by actuated prismatic or revolute joints, forming an open loop. When a serial robot is subjected to loading, the position of the farthest member, i.e., the end-effector, is subject to the cumulative deflections of all serial members. Since forces acting on the farthest member cause unfavorable bending and torsional moments in all serial members and therefore large deflections, the positioning error of the end-effector is significant. To improve the accuracy and stiffness of a serial robot, its members are designed to be rigid. However, this also increases the moving mass or inertia of the mechanism and causes a bulky design as well as a poor stiffness-to-mass and payload-to-mass ratio. Moreover, actuators of a serial robot are typically required to be mounted close to the joints along the serial chain and further increase the moving mass of the structure. As a result, the acceleration capability of serial mechanisms is limited, causing longer cycle and settling times in robotics applications.
A well-known mechanism of the serial type is the so-called SCARA robot, which includes three actuated revolute joints with parallel axes of rotation and one actuated prismatic joint. The device is able to position and orient an object in a cylindrical workspace with three translational degrees of freedom (x, y, z) and one rotational degree of freedom (rotation about z, i.e. the orientation), hence referred to as SCARA mobility. The inclination of the object, i.e. its rotation about axes x and y, remains constant with respect to the robot base for all poses of the mechanism. The fixed inclination makes this type of mechanism suitable for a wide range of industrial applications, such as handling, picking and placing, packaging or assembling. Another feature of the mechanism is that the axes of the actuated revolute joints are typically arranged in parallel to the direction of gravitational forces, such that the robot can be positioned manually in the x-y-plane by an operator, without any actuators or brakes engaged. This is useful when teaching the robot or resetting after a collision with the environment. Being a serial structure, however, a SCARA robot suffers from a poor stiffness-to-mass and payload-to-mass ratio, low natural frequencies and a limited speed and acceleration performance. Therefore, a need exists for a robot, which allows objects to be displaced in space with three or four degrees of freedom and retained inclination, and which provides high acceleration performance in combination with a light, rigid design and accurate positioning capability. A need also exists for such a robot to remain stationary under the influence of gravity and thus allow for manual positioning by an operator.
Relative to serial robots, parallel robots typically have an improved stiffness-to-mass and payload-to-mass ratio, better accuracy, superior dynamic properties and can move at higher speeds and accelerations. A parallel robot or mechanism has a plurality of links that form one or more closed loops, the links thereby sharing the load on the end-effector. Links of such a mechanism typically experience mostly compression or pulling forces, allowing the use of cheaper, lighter, and simpler material. Moreover, positioning errors of actuators are typically divided, thereby resulting in a high accuracy of the end-effector. In addition, actuators of parallel robots are often times mounted on or close to the base, which reduces the moving mass and allows for high end-effector accelerations. It also facilitates an easier design and an inexpensive integration of the actuators into the robot. Examples of parallel robots are illustrated in U.S. Pat. Nos. 6,497,548, 6,602,042, 6,648,583, and U.S. Patent Application 2004/0211284.
Further mechanisms of the parallel type have been presented, for example, in U.S. Pat. Nos. 4,976,582 and 6,516,681. Both devices provide SCARA mobility. Due to the rotationally symmetric arm arrangement of both devices, the workspace is relatively small. Moreover, as a result of the required suspension from a truss, the devices need significantly more floor space compared to traditional mechanisms such as SCARA robots, leading to a poor workspace-to-footprint ratio. The suspended mounting presents several further disadvantages: The truss needs to be rigid enough to prevent vibrations, which would impair the positioning accuracy of the end-effector. This causes additional cost and a bulky design. Furthermore, maintenance and repair work is more difficult than in traditional serial mechanisms, and the robot needs to be well-sealed to avoid contamination of underlying work pieces or conveyor belts. Devices of the aforementioned disclosures also do not allow for manual positioning as they collapse under the influence of gravity.
To reduce the footprint of parallel robots and avoid the disadvantageous suspension from a truss, alternative designs have been proposed. For example, the manipulator presented in U.S. Pat. No. 5,539,291 employs three arms interposed between a base and a moving element to displace an object in a cylindrical workspace with three degrees of freedom. Two arms operate in a horizontal plane and determine the radial distance and orientation of the moving element via a connecting rod and an attitude transmission member, which keeps the moving element at a fixed inclination with respect to the base. The third arm operates in a vertical plane and influences the axial position of the moving element in the cylindrical workspace. The functional association of the attitude transmission members with the connecting rods of the first two arms leads to a complicated and fragile design: In particular, the disclosed implementation with two opposing wheels and a cable is not only undesirable in terms of manufacturing cost and assembling, but also reduces the accuracy and rigidity of the mechanism. The mechanism is also not able to independently orient an object, i.e. rotate it about a vertical axis (z). The robot also collapses under the influence of gravity.
To simplify the design, alternative mechanisms similar to the one presented in U.S. Pat. No. 5,539,291 have been disclosed. For example, WO 02/22320 shows a manipulator to move an object in space with three arms. Two arms are mounted on a central column and rotatably actuated to move in horizontal planes while the third arm is actuated to operate in a vertical plane. Links connect the arms to the end-effector via joints, which lie on a common line of symmetry. This type of joint arrangement requires a lot of space at the end-effector and complicates its design. Moreover, it is not possible to place additional components such as work tools or actuators on the line of symmetry and close to the joints. This, however, would be desirable to reduce the moment of inertia about the line of symmetry. In one of the disclosed embodiments, the actuator of the third arm is mounted on and rotated by one of the other arms, causing additional inertia and asymmetric torque loads for the two arms. Whenever the end-effector is either at a great distance from or in close proximity to the central column, such an arrangement places the third arm in an unfavorable, asymmetric position relative to the other two arms and causes undesirable asymmetric stiffness, accuracy, and acceleration characteristics.
WO 02/058895 discloses a similar manipulator, which includes an additional linkage connecting the movements of the three arms such that the third arm remains in the middle between the other two arms. This results in an improved workspace-to-footprint ratio, which is comparable to that of serial robots of the SCARA-type. In both of the aforementioned disclosures, joints with three degrees of freedom such as ball-and-socket joints are used to connect links to the arms and to the end-effector. This, however, is not desirable in many applications due to backlash in the joints, friction and rapid wear. Joints must be manufactured with great precision in order to not impair the positioning accuracy of the end-effector. Moreover, precise ball-and-socket joints are expensive. A design with joints providing three degrees of freedom each also requires a large number of degrees of freedom of the passive joints per degree of freedom provided at the end-effector, leading to additional cost, weight, and inaccuracies. A solution for this particular disadvantage has been presented in U.S. Patent Application 2004/0211284.
Furthermore, mechanisms according to WO 02/22320 or WO 02/058895 include five, six, or even more links between the arms and the end-effector in order to transmit forces as well as moments acting on the end-effector via pure compression or pulling forces in the links. This increases the volume occupied by the mechanism and bears the risk of interferences between the links and other objects in proximity to the mechanism. Moreover, a high number of links is not needed particularly when external or inertial forces act on the end-effector at a point that is close by the longitudinal axes of the links. In such a case, said forces only cause small torsional or bending loads, which can easily be taken by the links. A lower number of links also reduces the weight and the number of required joints, which improves accuracy and saves cost of passive joints as well as links.
The described disadvantage also applies to the parallel mechanism disclosed in WO 03/066289. The device for positioning and orienting an object in space with at least three degrees of freedom comprises three arms, which rotate about the same axis and are driven by base-mounted actuators. Two of the arms control the position of the object in a horizontal plane normal to the axis, while the third arm influences its vertical position. Similar to SCARA robots, the inclination of the object can be kept constant. The coaxial arm arrangement, however, requires the circular, horizontal motion of the third arm to be translated into a vertical motion of the object, which is undesirable from a kinematics point of view: The velocity transmission ratio will vary significantly depending on the arm position and, along with the overall asymmetric arrangement, causes uneven and asymmetric stiffness, accuracy, and acceleration characteristics at the end-effector. Mechanisms of the disclosure also collapse under the influence of gravity, and thus do not allow for manual positioning by an operator.
Other parallel robots with coaxial arm arrangements have been proposed, for example in U.S. Pat. Nos. 4,946,337, 5,725,352 and 5,857,826. Manipulators of these disclosures have small footprints and retain the inclination of the end-effector, but do not allow for an axial displacement or an orientation of the supported object. Thus, they cannot replace traditional SCARA robots. However, a robot according to U.S. Pat. No. 5,857,826 always keeps the end-effector radially oriented, which is particularly useful in wafer handling applications. Another parallel mechanism with a small footprint has been presented in U.S. Pat. No. 4,407,625. The device comprises at least three arms and allows for an object to be displaced with at least three degrees of freedom. However, the inclination of the end-effector does not remain constant and the workspace is small compared to the entire robot volume.
Another major concern in many robotics applications and traditional robots is cable management. To connect various utilities such as power, sensors, or air pressure, utility lines must be routed along the moving serial structure of the mechanism, exposing such lines to significant stress and wear. To ensure operational reliability, custom-made power or utility lines are required, causing considerable extra cost.
Another concern particularly with existing serial mechanisms, such as SCARA or articulated robots, is the lack of scalability and modularity. To vary the output parameters, such as workspace size or shape, stiffness or accuracy characteristics, the entire serial structure including the actuators typically needs to be redesigned and replaced. Thus, serial robots do not allow for economies of scale when offered in various sizes, for example.
Therefore, a need exists for a simple and effective parallel robot for movement of an object in space with at least three or four degrees of freedom and retained inclination. A need further exists for a parallel robot that exhibits a large translational and rotational motion range of the end-effector in combination with small floor space requirements, thus resulting in a high workspace-to-footprint ratio. A need also exists to provide a fast mechanism with high acceleration capabilities and improved dynamic properties. Ideally, such a mechanism allows for the possibility to be equipped with means for compensating the influence of gravity, thus facilitating a manual positioning by an operator.
Moreover, a need exists for an accurate parallel robot that provides an improved stiffness-to-mass and payload-to-mass ratio. Ideally, stiffness, accuracy, and acceleration properties of the end-effector remain similar within the motion range of the end-effector. Furthermore, the mechanism should allow for simple cable management and improved operational reliability with reduced costs. The mechanism should also have a simple, robust, modular, and scalable design with no redundant joint freedoms as well as a low number of required links and joints to support the end-effector.